In beamforming data transmission from a transmitter unit to a receiver unit it is necessary to calculate the beams to the various scheduled receiver units in such a way that the reception of desired signals can be enhanced while simultaneously suppressing interfering signals. When applying a minimum means square error as the optimization criterion, the optimized antenna weights can be determined from the Wiener-Hopf solution as described in the algorithm below.
The system model for a base station 11 as depicted in FIG. 1 refers to a multi-antenna base station comprising antennas 13 A0 . . . AK-1 that are here, arranged as a uniformly spaced linear array (ULA). This, however, should only be understood as a non-limiting example that does not exclude other arrangements, e.g. circular arrangements, when adapting the formulas below.
The signal at the antenna array of the transmitter unit 111 of the base station 11 can be expressed as
      x    ⁡          (      t      )        =                    s        ⁡                  (          t          )                    ·      v        +                  ∑                  i          =          1                N            ⁢                                    u            i                    ⁡                      (            t            )                          ·                  η          i                      +          e      .      
Hereby, s(t) denotes the data signal to be transmitted to the user UE0 of interest while ui(t) represents the contribution of the interfering signal vectors from other equipments, i.e. UE1, . . . , UEN. v is the array propagation vector for the desired signal s(t) and ηi the array propagation vector for the interfering signal ui(t) from user equipment UEi. The array propagation can also be denoted the spatial signature of a signal represented by the relative phases at which the signal is received by the antennas of a receiver unit. e is a white noise component including, inter alia, thermal noise and outercell interference contributions.
For a base station comprising a uniformly spaced antenna array as described above, the array propagation vectors can be expressed as
            v      T        =          [              1        ,                  ⅇ                      j            ⁢                                          2                ⁢                π                            λ                        ⁢                          d              ·              sin                        ⁢                                                  ⁢                          θ              0                                      ,        …        ⁢                                  ,                  ⅇ                      j            ⁢                                          2                ⁢                π                            λ                        ⁢                          (                              K                -                1                            )                        ⁢                          d              ·              sin                        ⁢                                                  ⁢                          θ              0                                          ]        and            η      i      T        =                  [                  1          ,                      ⅇ                          j              ⁢                                                2                  ⁢                  π                                λ                            ⁢                              d                ·                sin                            ⁢                                                          ⁢                              θ                i                                              ,          …          ⁢                                          ,                      ⅇ                          j              ⁢                                                2                  ⁢                  π                                λ                            ⁢                              (                                  K                  -                  1                                )                            ⁢                              d                ·                sin                            ⁢                                                          ⁢                              θ                i                                                    ]            .      
vT denotes the vector of phase relations at the K antennas A0 . . . AK-1 of a uniformly spaced antenna array with distance d between the antenna elements, said relations relative to a reference antenna with normalized signature 1 when receiving the signal s(t) with wavelength λ and under an angle θ to the antenna plane. ηiT describes the corresponding vector for a signal ui(t).
When defining a vector w=[w0, w1, . . . , wK-1] of antenna weights, one conceivable criterion to achieve an optimization of the beamformer output signal wH·x(t) is to minimize the error ε(t) between said beamformer output signal and the desired signal s(t) (or a signal d*(t) that sufficiently close represents s(t) or at least correlates to a certain extent to the characteristics of s(t)). A minimization of ε2(t)=(d*(t)−wH·x(t)) can likewise be expressed by the equation ∇w(E{ε2(t)})=0.
When defining r=E{d*(t)·x(t)} and R=E{x(t)·xH(t)}, the solution of the equation above is wopt=R−1·r. Each of the K complex elements of the vector wopt describes amplitude and phase requirements for weighting the corresponding antenna element to form an optimized beam for transmission of the desired signal s(t) to the user equipment UE0.
The document WO 2005/060123 discloses an optimization for a MIMO network by selecting first and second sets of users, the second set not comprised in the first set, and adapting communication parameters for transmissions to said first and second set according to said parameters. In this way, communication with one or a few users can be optimized while network resources can be used in an efficient way also for other users.
The document US 2002/0094843 discloses a (sector) antenna beam-stearing using feedback from mobile stations for enhancing antenna beam steering effectiveness and efficiency.